While he was visiting the Syrian town of Aleppo in late 1994 and early 1995, Mohamed Atta met a young Palestinian woman called Amal. She worked in a planning bureau there, so she had plenty in common with Atta, who was studying town planning.

'I got the impression he was interested in her,' said Volker Hauth, a fellow student travelling with Atta at the time. Amal was attractive and self-confident. She observed the Muslim niceties, taking taxis to and from the office so as not to come into close physical contact with men on the buses. But, said Hauth, she was 'emancipated' and 'challenging'.

It seemed, too, that she was as interested in Atta as he was in her. Atta was Egyptian and Hauth last week recalled how Amal had teased his friend with one of those half-admiring, half-provoking asides that women reserve for men they find attractive. 'All Egyptians are Pharaohs,' she is said to have joked.

'He spoke about her back in the hotel. But he said she had a quite different orientation and that the emancipation of the young lady did not fit. He told this with regret,' said Hauth.

The story of Amal is the closest thing to romance in the austerely dutiful life of the pivotal figure in the inquiry into the attacks on New York and Washington.

Atta, 33, was the first of the alleged conspirators to enrol at the university on the outskirts of Hamburg which investigators believe was at the heart of the plot. It was he who remained throughout at the flat where at least three of the others lived. It was he who headed the university religious association to which they are all thought to have belonged. And on 11 September - investigators believe - it was Atta who led the attack on the World Trade Centre, piloting American Airlines Flight 11 into the north tower at 8.45am.

In many respects, though, he led not one life, but two. He repeatedly switched names, nationalities and personalities. If in Egypt, and later in the US, he was Mohamed Atta, then at the Technical University of Harburg, he was Mohamed el-Amir. For the university authorities, he was an Egyptian, yet for his landlord, as for the US authorities, he was from the United Arab Emirates. And while it is not hard to see Atta, whose face gazes out from the passport photograph released by the FBI, as that of the mass murderer of Manhattan, el-Amir was a shy, considerate man who endeared himself to Western acquaintances.

Such indeed was the gulf between the two that some people, notably his father, insisted last week that Mohamed Atta's identity must have been stolen by the hijackers' leader. That view was given some credibility by a German press report, not denied by the government, that he and two other Hamburg suspects reported in 1999 that their passports had been stolen. However, the same report quoted the Interior Ministry as saying that the reason the three men did so was to obtain new passports, free of stamps that might have jeopardised their chances of obtaining US visas.

What The Observer's investigation into his past has revealed is that Mohamed el-Amir Awad el-Sayed Atta, to give him the full name under which he registered in Germany, underwent a visible process of radicalisation. He may have led a double life, but he was no 'sleeper'. Indeed Mohamed el-Amir, the student, was much more overtly fundamentalist than the shadowy Mohamed Atta.

Atta was born at Kafr el-Sheikh in the Nile Delta and brought up in the slightly down-at-heel Cairo suburb of Giza. His father was a lawyer and he studied architecture at the university of Cairo between 1985 and 1990.

Hauth, who travelled with him to Egypt, observed last week that Atta came from precisely that traditionally minded sector of the intelligentsia which was most outraged, and prejudiced, by the opening to the West that President Anwar Sadat initiated before his assassination in 1981.

When Atta arrived in Harburg 11 years later to study for the equivalent of an MSc in town planning, he left behind him a country once again drifting into turmoil as Islamic fundamentalists mounted a campaign to overthrow the government. In October 1992, the month Atta enrolled, it was reported from Cairo that terrorists would henceforth be tried before military courts. That decision set the stage for a brutal trial of strength marked by savage attacks on the one hand and, on the other, by widespread torture and the imprisonment of thousands of people without charge or trial.

Atta made no secret of where his sympathies lay. He had graduated from a faculty that was a hotbed of fundamentalist agitation and gone on to join the Engineers Syndicate, one of three professional associations controlled by the Muslim Brotherhood.

Hauth said his acquaintance was appalled by what he saw as the creation of a new class of Egyptian 'Fat Cats'.

'One of the main points of his critique was the contrast between a few rich people and the mass of people with barely enough to survive'.

In Germany, Atta was soon able to turn his architectural training, and specifically his drawing skills, to advantage. Just two months after his arrival, he secured a part-time job with Plankontor, a planning consultancy in the trendy Hamburg district of Ottensen.

Helga Rake, one of the partners at Plankontor, remembered him as 'introverted and very reserved', but also as 'flexible' and 'very conscientious'.

She added: 'He prayed in the office. We'd never had anyone do that before.' At midday, the man they knew as Mohamed el-Amir, would break off whatever he was doing to kneel down beside his drawing board. 'He was very critical of capitalistic, Western development schemes,' said another partner, Matthias Frinken. 'He was critical of big hotels and office buildings.'

But there was little to suggest that Atta was any different from millions of other devout, peaceful, religiously conservative yet socially aware Muslims.

Professor Dittmar Machule, who supervised his thesis and also knew him as el-Amir, said: 'At the beginning, we spoke often about how religions can co-exist. He was very intellectually engaged with this problem.'

A photograph taken of Atta on a student trip to Istanbul in the summer of 1994 shows him clean-shaven. But a year later, when he returned to Cairo, Atta had acquired that distinctive beard which fringes the chin and leaves the upper lip free of hair which, in North Africa, is usually the sign of a committed fundamentalist.

It is at this point that odd gaps begin to open up in his life and the first evidence appears of his dissembling. Helga Rake at Plankontor said that he was absent for half of 1995 and that he said he had taken time out to make the pilgrimage to Mecca and return home. But Hauth said that neither on their subsequent visit to Cairo, nor at any time over the next few months, did the Egyptian mention a pilgrimage. This is all the more extraordinary given the German's keen interest in religion. 'It was the very basis of our relationship', he said.

In June 1997, Atta was laid off by Plankontor. The partners had bought a CAD system and his draughtsmanship was not needed. 'When he was given his last sum of money, he got too much from us and he sent it back,' recalled Frinken. 'He said that he hadn't earned it and he didn't want any more'.

Machule said Atta then took a long break from his studies. The recollections of others show it could only have been in the period from the end of the academic year in 1997 to the start of the academic year in 1998 - a gap of 15 months which the Egyptian explained to his professor as being for family reasons.

It is striking that Atta's absence coincided with an upsurge in violence directed against foreigners by an extremist group, Jama'ah al-Islamiyah, known to be linked to Osama bin Laden. It is even more striking that the victims of the first such attack, on a tourist bus in Cairo, which left nine dead and 11 injured, were Germans.

By the time Atta returned to Hamburg he was a changed man. Hannelore Haase, who owned the shop at the corner of the street where Atta shared a flat with two other Arab men remembered all three wearing traditional garb of baggy trousers and flowing kaftans. Chrylla Wendt, Machule's assistant, said he now had a thick, bushy beard. 'He was more serious,' said the professor.

Hauth, who lost contact with Atta after he left the university at the end of 1995, knew a man who could even laugh at jokes about Arab dictators. But Wendt said: 'I cannot remember him smiling.'

She had plenty of opportunity to study Atta at close quarters, for she had agreed to go through his thesis with him, correcting his German. Starting in June 1999, they met 'at least once a week' in her narrow office and sat side by side at her desk.

But when the time came to look at the last chapter, Atta refused to go through it with her and Wendt believes he had found their physical intimacy unbearable. The thesis was finished. But before it was submitted, Atta slipped in an additional page at the front. It had on it a verse from the Koran.

'Say. My prayer and my sacrifice and my life and my death are [all] for Allah, the Lord of the worlds.'

Machule shrugged it off as the idiosyncrasy of a devout man. His thesis was what mattered, and it was brilliant. Atta got a 1.0 - the highest possible mark.

Wendt remembered how, when the examiners had finished their deliberations, Machule walked out to congratulate Atta. An outside, female examiner followed suit and extended her hand. Atta refused to take it.

Although he remained enrolled, no one on campus seems ever to have seen him again. He next reappears this year making an unexplained 10-day visit to Spain. The Observer has seen hotel records which confirm that Atta spent at least one night in the eastern resort town of Salou in mid-July.

The FBI said that Atta flew to Madrid's Barajas airport from Miami on 9 July. His first step on arrival was to pick up a rental car which he had previously reserved over the internet.

Atta spent his last night in the Montsant Hostal in Salou, where he paid with his Visa card and registered under his own name. His Hyundai Accent car was returned to Madrid airport on 18 July with some 1,250 miles on the clock.

On 16 August, back in Florida, he rented a single-engined plane from a company in Palm Beach. He made a test flight to demonstrate his competence and then returned twice more, each time with a different passenger.

In the minds of all but the most cynical or sadistic terrorists, there has to be an element of wilful schizophrenia - a readiness to murder people in the name of humanity. But in the mind of Atta, that wilful schizophrenia seems to have attained extraordinary proportions.

He cared deeply about people. It is not just that he cared about the Muslim poor. He even cared about the next American to rent his hire car. Brad Warrick, of Warrick's Rent-a-Car in Pompano Beach, Florida, said that Atta called him to say the car's oil light was on. When he returned it on 9 September, Atta reminded him about the light.

Unconsciously echoing the many Germans who experienced Atta's consideration, Warrick said: 'The only thing out of the ordinary was that he was nice enough to let me know that the car needed an oil change.'

Yet when that same man seized the controls of American Airlines Flight 11 two days later and aimed it at the World Trade Centre, he seems to have been able to dismiss from his mind the fact that the building, like the plane, was full of people he was about to send to a terrifying death: kind-hearted, middle-aged PAs such as Chrylla Wendt, dynamic young professionals like Volker Hauth and, in the World Trade Centre of all places, lots of challenging, emancipated women, just like Amal.

Additional reporting by Giles Tremlett in Salou.

Mohamed Atta

Age 33

Born At Kafr el-Sheikh in Egypt's Nile Delta and brought up in Cairo. Father a lawyer

Education Studied architecture in Cairo and town planning in Hamburg. Trained as a pilot in Florida